Hello,
I've recently ran into an issue with delivering mails to a
Mimecast-hosted domain.
I do not have delivery issues to any other MX, there are no blocklist
entries, and SPF, DKIM and DMARC are working as expected. I've just
confirmed this again via mail-tester.com and dmarctester.com.
A domain one of my users has been sending to for the past 25 years just
earlier this week switched to Mimecast, and since then the majority of
mails get blocked with "550 DKIM Sender Invalid - envelope rejected",
other mails make it through. So far, I see no pattern what might trigger
the 550. Senders and recipient differ and the choice of mail client
seems to make no difference either. As all of the outgoing mails use the
same MX, it's unlikely DKIM signing works on some and not on others
(plus then I would also see rejections from other sites).
For now, I assume it's a problem on the recipient's end, perhaps some
misconfiguration. That particular error message is not listed on
https://community.mimecast.com/s/sender-feedback either.
Anyone has insight how to nail down that issue?
Thanks
Florian
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